Robot arms comprising a plurality of arm sections pivotally linked together, wherein relative motion of the arm sections is brought about by hydraulic linear actuators, are known, for example, from U.S. Pat. No. 4,693,663 (Brenholt et al.) and similar arrangements are known from U.S. Pat. No. 3,792,782 (Melton) and U.S. Pat. No. 5,060,532 (Barker) where the arm is a load-carrying boom. In each of these cases the hydraulic actuators act directly on the respective arm sections. Linkage mechanism are also known whereby a hydraulic actuator acts via a linkage to cause bending of a hinge mechanism, for example in the concrete mixer chute of U.S. Pat. No. 4,458,800 (Christenson) and in the surgical table of U.S. Pat. No. 4,706,511 (Houston).
Generally according to the present invention there is provided a manipulator with a plurality of hydraulically operable joints, each joint comprising a first arm member pivotally linked at a center axis to a second arm member, each arm member being at least in part of hollow box section with spaced-apart walls, and the pivotal linkage being provided by two spaced-apart pivotal links between corresponding walls of the arm members, a linear hydraulic actuator pivotally connected at one end to the first arm member and at the other end to two linking members, the first linking member being pivotally connected to the first arm member, and the second linking member being pivotally connected to the second arm member, wherein the first linking member comprises two parallel spaced-apart link bars each pivotally connected at one end to the hydraulic actuator, and each pivotally connected at the other end to a respective wall of the first arm member by respective spaced-apart pivotal links on a common axis, the manipulator also comprising at least one cable extending along the first arm member and extending to the second arm member passing substantially through the center axis of the joint.
The term manipulator is usually taken to refer to a mechanical arm which is controlled by an operator, but in this context it should also be taken to refer to robot arms, which operate automatically. The joint is a compact design which allows considerable freedom of movement. The term cable should be taken to encompass electrical cables and hydraulic hoses. Because the cables pass through the centre axis of the joint their length does not need to change as the joint bends, so that no reeling of cables or rotary cable coupling, or protruding slack cable is required. Because the arm members are hollow, the cables can therefore extend within the arm members, and so are enclosed along their whole length.
Preferably the distance between the two pivotal connections to the first linking member is greater than the distance between the two pivotal connections to the second arm member. This increases the angle through which the joint can bend for a given stroke of the linear actuator; and enables the variation in torque, as the angle varies, to be reduced (the force exerted by the actuator being constant).